1. Field of Invention
This invention relates to security tags and, more particularly, to localizing security tags and security tag readers.
2. Description of Related Art
U.S. Patent Publication No. 2002/0145563 A1, filed on Jan. 26, 2001 by Kane, discloses the use of time of arrival information and phase difference information in backscatter signals for precisely localizing a subject, and precisely monitoring the gait of the subject. In the Kane system the backscatter signals are produced by a plurality of RFID tags positioned at different locations on the body of the subject to be monitored. Additionally, Kane teaches a plurality of receiving antennas for receiving the signals from the tags on the body of the subject, and performing separate phase difference calculations.
U.S. Pat. No. 7,009,561, issued on Mar. 7, 2006 to Menache, discloses an RF motion tracking system including a stationary RF transmitter and a plurality of stationary RF receivers defining sensors placed at known location to define a capture zone. A plurality of RF tags is placed on an object being tracked to serve as marker tags. The marker tags backscatter the signal from the stationary RF transmitter. The sensors determine the relative phases between the signal received from the stationary RF transmitter and the signals received from the marker tags. The relative phase information determined at the sensors is processed to determine the respective positions of the marker tags. However, in Menache the marker tags are distributed over a number of items, and are thus not disposed on a single tag device. Furthermore, the marker tags in the Menache system do not maintain a known geometric relationship with respect to each other.
U.S. Patent Publication No. 2005/0172215 A1, filed on Jan. 14, 2005 by Squibbs, teaches that multiple tags can be embedded in an object in a system for determining the location of the object. The object can be, for example, a sheet of paper. At least one of the tags stores location data concerning a relationship between the object and one or more of the embedded tags. The tags taught by Squibbs contain location data which permit a determination where the object belongs when the data is scanned and displayed. The relative positions of the embedded tags are not used to determine where the object is located.
Several references disclose localizing items using a plurality of receivers. In these systems information such as phase differences, differences in round trip travel times and differences in field strength are determined because the different receivers receive signals from the items at different distances and/or directions.
For example, U.S. Pat. No. 6,380,894, issued on Apr. 30, 2002 to Boyd, discloses an asset management radio location system that uses time of arrival differentiation from a plurality of distributed tag transmission readers to locate a tag fixed to an item within a monitored environment. An object location processor is coupled to the distributed readers over signal transport paths having different transport delays, and carries out the time of arrival differentiation of the transmissions from a tag to determine where its object is located. U.S. Pat. No. 6,400,139, issued on Jun. 4, 2002 to Khalfin, teaches placing one or more probe sensors on an object to be tracked within a volume, and placing a plurality of witness sensors within the volume. The output of the witness sensors is used to compute the position and orientation of the probe sensors. U.S. Patent Publication No. 2003/0002033 A1, filed on Jun. 27, 2001 by Boman, discloses three sensor sets for detecting the location of a radiation source. Each sensor set in Boman can contain a pair of sensors for detecting the location of two different radiation sources simultaneously. U.S. Pat. No. 6,549,004, issued on Apr. 15, 2003 to Prigge, discloses a system wherein a plurality of beacons located at known positions generates coded magnetic fields, and a magnetic sensor measures a sum field. The sum field is decomposed into component fields to determine the sensor position and orientation. These references teach a single source being localized by a plurality of receivers.
Additionally, U.S. Pat. No. 6,754,609, issued on Jun. 22, 2004 to Lescourret, discloses a device for measuring the position and orientation of a mobile object with respect to a fixed structure. In the Lescourret system a first assembly of coils emitting magnetic fields is secured to the fixed structure, and a second assembly of coils for receiving magnetic fields is secured to the object, forming a sensor. The position and orientation of the mobile object is determined according to the sensor readings of the emitted fields. What is needed is a system and method for more accurately localizing tags and tag readers without the use of physical antenna arrays.
All references cited herein are incorporated herein by reference in their entireties.